text: Rain gardens are designed to capture the initial flow of stormwater and reduce the accumulation of toxins flowing directly into natural waterways through ground filtration. Natural remediation of contaminated stormwater is an effective, cost-free treatment process.  Directing water to flow through soil and vegetation achieves particle pollutant capture, while atmospheric pollutants are captured in plant membranes and then trapped in soil, where most of them begin to break down. These approaches help to diffuse runoff, which allows contaminants to be distributed across the site instead of concentrated. The National Science Foundation, the United States Environmental Protection Agency, and a number of research institutions are presently studying the impact of augmenting rain gardens with materials capable of capture or chemical reduction of the pollutants to benign compounds.
  The primary challenge of rain garden design is predicting the types of pollutants and the acceptable loads of pollutants the rain garden's filtration system can process during high impact storm events. Contaminants may include organic material, such as animal waste and oil spills, as well as inorganic material, such as heavy metals and fertilizer nutrients. These pollutants are known to cause harmful over-promotion of plant and algal growth if they seep into streams and rivers. The challenge of predicting pollutant loads is specifically acute when a rain event occurs after a longer dry period. The initial storm water is often highly contaminated with the accumulated pollutants from dry periods. Rain garden designers have previously focused on finding robust native plants and encouraging adequate biofiltration, but recently have begun augmenting filtration layers with media specifically suited to chemically reduce redox of incoming pollutant streams. Certain plant species are very effective at storing mineral nutrients, which are only released once the plant dies and decays. Other species can absorb heavy metal contaminants. Cutting back and entirely removing these plants at the end of the growth cycle completely removes these contaminants. This process of cleaning up polluted soils and stormwater is called phytoremediation.